The present invention relates to belt drive continuously variable speed transmissions (CVT) comprising primary and secondary pulleys each consisting of two sheaves, with a metallic belt or chain wound around the pulleys. More particularly, it relates to arrangements in CVT's of the above-mentioned type for carrying thrusts exerted on the pulleys.
Recently, from viewpoints of fuel consumption rate and others, automatic transmissions of the type incorporating a belt drive continuously variable speed transmission therein have attracted attention as a transmission for use in motor vehicles.
The applicant has already proposed a belt drive continuously variable speed transmission, wherein, as disclosed in Japanese Laid-Open Official Gazette for Patent Application No. 62-13853, the arrangement is such that axial forces corresponding to transmitted torque are imparted to pulleys by means of pressure adjusting cam mechanisms and the effective diameter of the pulleys ar adjusted by ball screw mechanisms.
In this belt drive continuously variable speed transmission, thrust roller bearings are interposed between movable sheaves and ball screw mechanisms, and other thrust roller bearings are interposed between flange portions provided at shaft ends and the opposite ends of the ball screw mechanisms, belt squeezing forces from the pressure adjusting cam mechanisms and the ball screw mechanisms being imparted via these bearings, the axial forces being carried by the tensile force exerted by the shaft.
In the above-mentioned belt drive continuously variable speed transmission, the race position of the thrust bearings is uniquely defined by the precision on the side of the sheave, or the precision of the ball screw mechanisms, so that th ethrust bearings are directly influenced by such supporting members. Therefore, there is a problem in that dispersion of the precision of the supporting members may cause the rollers to abut against the race on a single side, with concentrated loads being applied to part of the bearings, which may largely reduce the durability of the bearings.
Since, in the belt drive continuously variable speed transmission, the movable sheaves are slidably guided on shafts while being influenced by large biasing forces originating from the tension of the belt, it is extremely difficult to maintain the supporting members. i.e. ball screw mechanisms, together with the movable sheave backsides and the flange front sides at a high positional accuracy. Further, in the supporting mechanism comprising a closed loop for carrying the axial forces acting on the pulleys within the shafts, the supporting structure does not have any relief portions so that the above-mentioned positional accuracy will prominently influence the performance of the bearings.